Recently, with the rapid progress in recording technologies, it has been possible to obtain high-definition image quality substantially equal to that of silver halide photography. Accordingly, ink-jet recording apparatuses, that form images by an ink-jet recording system, have been widely used as image forming apparatuses.
For such ink-jet recording apparatuses, improvement in image formation speed has been strongly desired while maintaining image quality. To increase the image formation speed, it is effective to use an ink-jet recording apparatus using a line-head recording system (hereinafter, also referred to as a “line-head-type ink-jet recording apparatus”). When an image is formed at high speed with line-head-type ink-jet recording apparatus, however, a recording medium, such as paper, is conveyed by an output roller pair to the outside before an ink permeates into the recording medium. In this case, the ink may adhere to the output roller (offset). This is liable to cause an image defect. To reduce the occurrence of offset, it is conceivable that the amount of the ink ejected is reduced and that the permeability of the ink to the recording medium and drying properties of the ink are enhanced. In the former situation, it may be difficult to form an image having a desired image density. In the latter situation, when an image is formed after the ink has not been ejected from a recording head for some time, the failure to eject an ink may be caused by, for example, a change in the viscosity of the ink.
In light of the foregoing, an ink for use in ink jet recording is reported as an ink capable of inhibiting the clogging of the recording head and failure to eject even if the solidification of the ink, which may cause the failure to eject ink droplets from a recording head, occurs. The ink contains a pigment, water, and an water-soluble organic solvent that contains polyethylene glycol and/or diglycerol, and a content of polyethylene glycol and/or diglycerol being 10% by weight or more.
In the ink for ink-jet recording, however, the polyethylene glycol and diglycerol contained in the ink each has a high viscosity as a solvent itself. Thus, a change in the viscosity of the ink due to the evaporation of water is liable to occur. In the case where such an ink is used for, in particular, a line-head-type ink-jet recording apparatus including a plurality of recording heads, a significant increase in the viscosity of the ink due to the evaporation of water is observed.